Microcavity plasma devices and arrays have been developed and advanced by researchers at the University of Illinois, including inventors of this application. Devices and arrays have been fabricated in different materials, such as ceramics and semiconductors. Arrays or microcavity devices have also been fabricated in thin metal and metal oxide sheets. Advantageously, microcavity plasma devices confine plasma in cavities having microscopic dimensions and require no ballast, reflector or heavy metal housing. Microcavities in such devices can have different cross-sectional shapes, but generally confine plasma in a cavity having a characteristic dimension in the range of about 5 μm-500 μm.
FIG. 1 is a schematic diagram of a single plasma jet device of the prior art. A gas supply 10 provides a gas flow through a tube 12 that includes a nozzle exit 14. A power supply 16 powers electrodes 18 and 20, which stimulate plasma generation in the tube 12 and a plasma jet 22 is emitted from the nozzle exit 14. The tube is generally cylindrical and has a typical diameter of several millimeters. The electrodes in such prior devices are generally on the outside of the tube. Several kV are required to produce the plasma jet as a result of the tube diameter, and the tube structures are not readily bundled because they are heavy, and an ensemble of even a few tubes is bulky.
Such previous plasma jet technologies have a number of limitations. First, tubing is often used with diameters that have been large for a single jet, typically on the order of millimeters (mm). Producing a jet in tubing of such a diameter requires very high voltages (many kV) and a high gas flow (typically, several standard liters per minute (SLM)). Another difficulty arises if one wishes to make a plasma source that covers an area as large as possible. “Bundling” a number of tubes together, each of which is itself bulky and heavy, is difficult, and is inconvenient for many applications. For this reason, typical multiple jet assemblies developed in the past are not practical.
Burton et al., “Initial Development of the Microcavity Discharge Thruster,” 31st International Electric Propulsion Conference, University of Michigan, USA September 20-24, (2009) & Chadenedes et al., “Advances in Microcavity Discharge Thruster Technology,” American Institute of Aeronautics and Astronautics (2010) disclose microjet devices formed in two thin foil layers of Al/Al2O3. The layers defined a nozzle in a bowl shape that varied from linear to parabolic in cross-sectional geometry. Chadenedes et al. discloses in FIG. 1 a microjet device formed from two metal/metal oxide electrodes. The electrodes in FIG. 1 form an aligned cylindrical surface (the surfaces are flush) and the micronozzle in FIG. 1 is a separate structure from the metal/metal oxide electrodes. The ability to fabricate such a separate nozzle is not described, and the reliability and manufacturability of such a nozzle is not clear. FIG. 2 of Burton et al., describe a “bell shaped” nozzle microjet devices formed in the metal and metal oxide electrodes. The fabrication of a micronozzle is described as a chemical etching after the metal and metal/oxide foils are combined. The papers, as indicated in Burton et. al are directed to simulations and “proof-of-concept” efforts to produce a thruster for spacecraft. In FIG. 6 of Burton et al, a boron nitride shroud was used at the nozzle and a polyimide coating was used with two electrodes that form an abrupt change in size with a smaller microcavity having a smaller cylindrical microcavity. FIG. 7 of Burton et al shows the bowl/bell shaped nozzle. The “supersonic nozzle” of FIG. 1 or is shown as not being unitary with either of the electrodes. Moderate efficiencies are reported on page 7 for providing thrust by Burton et al., which proposes improvements might be achieved by “by coupling to a micronozzle for which performance has been characterized” and “the efficiency of the thruster can be further increased with improvements to the manufacturing process.” No solution is proposed in Burton for coupling a nozzle with optimal contours that are discussed on page 7 as being desirable.